Was running in Snow Leopard. Got my mail migrated to Apple Mail and haven't had to open Eudora since.
I was periodically encountering Eudora corrupting mailboxes when moving a bunch of mail. Particular problems were huge HTML-formatted emails from a well-known brokerage, and virtually any RTF message from Outlook. Eudora would crash, corrupt a mailbox and not tell me, then not be able to rebuild the index properly. Trying to delete corrupted emails via Eudora only served to lock in the corruption.
I spent weeks - literally weeks - with a text editor and Python sorting out the mess. Lost a bunch of emails (most of which I'd honestly not looked at in years and won't miss). I was able to do a regex search for lines including ???@??? where the first "?" wasn't the first character in the line. When I found that, I knew that (at least) the prior message was corrupt. But found so many I had to figure out how to automate the process in Python.
De-corrupting mailboxes was an iterative process. Test in Python, fix with text editor, make the Python code a little smarter.
Previous attempts at repair had left me with thousands of dupe messages, too. Some otherwise identical emails had different dates, due to date corruption problems. I had to sort out all the dupes.
Once I handled corruption and dupes, I had the problem of thousands of identically-dated "sent messages". Eudora keeps only minimal date data with sent mail, and when it's wrong, there's nothing in the headers to recover.
I was fortunate enough to have a backup of most of those thousands of incorrectly-dated sent messages dating from before the day-of-corruption. I wrote a python script that hashed each bad-date message, then looked through an index of hashes of the backed-up messages to see if it could find a correct message to swap in place. I thus rebuilt the sent messages mailbox.
After all of this, I tried Eudora Mailbox Cleaner to move everything over to Apple Mail. This worked just great. I had to figure out I had one mailbox path that wouldn't convert, but everything else went fantastic.
From Eudora, I'll mostly miss personalities, of which I was making intense use. Personalities don't show up in Apple Mail, nor in its available rules.
Here's the big takeaway from me: though I loved Eudora, stayed on it for years and years, it's now terribly broken. If it's not crashing and corrupting your mailboxes every day, it's - at a minimum - eventually going to hold you back from having a currently-patched operating system - which is pretty problematic. It's been holding me back from getting an OS that supports full-disk encryption. I'd mostly stopped taking my Mac Book Pro out of the house for fear of what happens if it gets stolen out of the trunk. Right. I was doing that 'cause I just loved Eudora so darn much. Something wrong here.
I spent so much time dealing with the corruption, sweating over all the dupes, the lost mail, the date problems, the constant crashing, the wondering when, whether, how I'd find something to replace it, that when I finally got done and off it, it felt like I'd escaped.
It's like being in a toxic relationship with someone you love. Though you'd like to, you can't fix it, and you really should just go.
You can love this software too much. Take it from someone who has. It's supposed to help YOU with the email, not the other way around. I think It's time to let Eudora rest in peace. Perhaps if we could shake off the spectre of someone building "someting just as good", someone could build "something new we haven't thought of yet" - which Eudora once was.
I'm a little sad to say it, but I'm very happy to finally be free.